In a direct injection internal combustion engine, a fuel injector is provided to deliver a charge of fuel to a combustion chamber prior to ignition. Typically, the fuel injector is mounted in a cylinder head with respect to the combustion chamber such that its tip protrudes slightly into the chamber in order to deliver a charge of fuel into the chamber.
One type of fuel injector that is particularly suited for use in a direct injection engine is a so-called piezoelectric injector. A piezoelectric injector 12 and its associated control system 14 are shown schematically in FIG. 1.
The piezoelectric injector 12 includes a piezoelectric actuator 16 that is operable to control the position of an injector valve needle 17 relative to a valve needle seat 18. The piezoelectric actuator 16 includes a stack 19 of piezoelectric elements, having the electrical characteristics of a capacitor. The stack 19 may be charged or discharged by application of a differential voltage to positive and negative terminals of the actuator 16, which causes the stack of piezoelectric elements to expand or contract. The expansion and contraction of the piezoelectric elements is used to vary the axial position, or ‘lift’, of the valve needle 17 relative to the valve needle seat 18.
The piezoelectric injector 12 is controlled by an injector control unit 22 (ICU) that forms an integral part of an engine control unit 24 (ECU). The ICU 22 typically comprises a microprocessor 26 and memory 28. The ECU 24 also comprises an injector drive circuit 30, to which the piezoelectric injector 12 is connected by way of first and second power supply leads 31, 32. In a so-called ‘discharge to inject’ injector, in order to initiate an injection event, the injector drive circuit 30 causes the differential voltage applied to the injector 12 to transition from a high voltage (typically 200 V), at which no fuel delivery occurs, to a relatively low voltage (typically −55 V), which causes the valve needle 17 to lift away from the valve needle seat 18.
Like any circuit, faults may occur in a drive circuit. In safety critical systems, such as diesel engine fuel injection systems, a fault in the drive circuit may lead to a failure of the injection system that could consequentially result in a catastrophic failure of the engine. Diagnostic systems for detecting short circuit faults in piezoelectric actuators of piezoelectric injectors are disclosed in applicant's co-pending patent applications EP1843027, EP1860306, EP1927743, and EP07252534.8, EP07254036.2 the contents of each document being incorporated herein by reference.
Of particular relevance to this application is co-pending application EP 07252534.8, which describes a diagnostic method for detecting three main types of short circuit fault, these are:
i) a short circuit between the terminals of a piezoelectric actuator; otherwise referred to as a ‘stack terminal’ short circuit;
ii) a short circuit from the positive terminal of a piezoelectric actuator to a ground potential; the positive terminal is also referred to as the ‘high’ terminal, and this type of short circuit is generally referred to as a ‘high side to ground’ short circuit; and
iii) a short circuit from the negative terminal of a piezoelectric actuator to a ground potential; the negative terminal is also referred to as the ‘low’ terminal, and this type of short circuit is generally referred to as a ‘low side to ground’ short circuit.
Referring also to FIG. 2, this shows the injector drive circuit 30 described in EP 07252534.8. The injector drive circuit 30 comprises an injector bank circuit 33, in which a pair of piezoelectric injectors 12a, 12b are connected. It should be appreciated that although the respective injectors 12a, 12b are shown as integral to the injector bank circuit 33 in FIG. 2, in practice the injector bank circuit 33 would be remote from the injectors 12a, 12b and connected thereto by way of power supply leads.
The drive circuit 30 includes three voltage rails: a high voltage rail VH (typically 255 V), a mid voltage rail VM (typically 55 V), and a ground voltage rail VGND (i.e. 0 V). The drive circuit 30 is generally configured as a half H-bridge with the mid voltage rail VM serving as a bi-directional middle current path 34. The injector bank circuit 33 is located in the middle current path 34 of the drive circuit 30 and comprises a pair of parallel branches 33a, 33b, in which the piezoelectric actuators 16a, 16b (hereinafter referred to simply as ‘actuators’) of the injectors 12a, 12b are respectively connected. The injector bank circuit 33 further comprises a pair of injector select switches SQ1, SQ2 connected in series with the respective injectors 12a, 12b in the respective branches 33a, 33b of the injector bank circuit 33. Each injector select switch SQ1, SQ2 has a respective diode D1, D2 connected across it. The injector bank circuit 33 is located between, and coupled in series with, an inductor L1 and a current sensing and control means 35.
A voltage source VS is connected between the mid voltage rail VM and the ground rail VGND of the drive circuit 30. The voltage source VS may be provided by the vehicle battery (not shown) in conjunction with a step-up transformer (not shown), or other suitable power supply, for increasing the voltage from the battery to the required voltage of the mid voltage rail VM.
A first energy storage capacitor C1 is connected between the high and mid voltage rails VH, VM, and a second energy storage capacitor C2 is connected between the mid and ground voltage rails VM, VGND. The first capacitor C1, when fully charged, has a potential difference of about 200 Volts across it, whilst the potential difference across the second capacitor C2 is maintained at about 55 Volts. A charge switch Q1 is located between the high and mid voltage rails VH, VM, and a discharge switch Q2 is located between the mid voltage and ground rails VM, VGND.
In essence, the drive circuit 30 comprises a charge circuit and a discharge circuit. The charge circuit comprises the high and mid voltage rails VH, VM, the first capacitor C1 and the charge switch Q1, whereas the discharge circuit comprises the mid and ground rails VM, VGND, the second capacitor C2 and the discharge switch Q2. The charge switch Q1 is operable to connect the injectors 12a, 12b to the first capacitor C1 causing a current to flow in the charge circuit, in the direction of the arrow ‘I-CHARGE’, to charge the actuators 16a, 16b to a known voltage. The diodes D1, D2 connected across the injector select switches SQ1, SQ2 allow the injectors 12a, 12b to charge in parallel when the charge switch Q1 is closed. To initiate an injection event from a selected injector 12a or 12b, a current is caused to flow in the discharge circuit, in the direction of the arrow ‘I-DISCHARGE’. This is achieved by closing both the discharge switch Q2 and an injector select switch SQ1, SQ2 to connect the selected injector 12a or 12b to the second capacitor C2.
The drive circuit 30 further includes a resistive bias network 36 connected between the high voltage rail VH and ground rail VGND, and intersecting the middle circuit branch 34 at a bias point PB. The restive bias network 36 is used to determine the voltage VB at the bias point PB in order to detect short circuit faults on the injectors 12a, 12b. 
The resistive bias network 36 includes first, second and third resistors R1, R2, R3 connected together in series. The first resistor R1 is connected between the high voltage rail VH and the bias point PB, and the second and third resistors R2 and R3 are connected in series between the bias point PB and the ground rail VGND. The first, second and third resistors R1, R2, R3 each have a known resistance of a high order of magnitude, typically of the order of hundreds of kiloohms. For convenience, R1, R2 and R3 are used herein to refer to both the resistors and to the resistances of the resistors R1, R2, R3.
To determine the voltage VB at the bias point PB, a voltage VS is sampled between the second and third resistors R2, R3 in the resistive bias network 36 using an analogue to digital (A2D) module of the microprocessor 26 (FIG. 1). The resistors R2 and R3 form a potential divider, and so the voltage VB at the bias point PB is calculated according to equation 1 below.
                              V          B                =                                            V              S                        ⁡                          (                                                R                  2                                +                                  R                  3                                            )                                            R            3                                      1      
To detect high and low side to ground short circuit faults on the injectors 12a, 12b, a so-called ‘unselected voltage reading’ technique can be employed. The unselected voltage reading technique involves determining the voltage VB at the bias point PB with neither of the injectors 12a, 12b selected, i.e. with both injector select switches SQ1, SQ2 open. When both injector select switches SQ1, SQ2 are open, a voltage VBpred at the bias point PB can be predicted from the high rail voltage VH, and the value of the resistors R1, R2, R3 in the resistive bias network 36, according to equation 2 below.
                              V          Bpred                =                                            V              H                        ⁡                          (                                                R                  2                                +                                  R                  3                                            )                                                          R              1                        +                          R              2                        +                          R              3                                                  2      
In the event that either of the injectors 12a or 12b has a high side to ground short circuit, then this short circuit behaves as a resistor connected in parallel with the resistors R2 and R3 in the resistive bias network 36. If the voltage VB is measured at the bias point PB when there is a high side to ground short circuit, then the measured voltage will be lower than the predicted voltage VBpred according to equation 2 above. However, if one of the injectors 12a, 12b has a low side to ground short circuit, then the measured voltage at the bias point PB will be higher than the predicted voltage VBpred according to equation 2, and will depend upon the inherent resistance of the low side to ground short circuit. Hence, by measuring the voltage at the bias point PB and comparing it to the predicted voltage VBpred according to equation 2 above, high and low side to ground short circuit faults on the injector bank 33 can be detected.
Stack terminal short circuits can also be detected using the resistive bias network 36. If an injector 12a, 12b has a stack terminal short circuit, then it will not hold its charge following a charge event on the bank 33. Instead, the injector 12a, 12b will discharge through the stack terminal short circuit at a rate governed by the inherent resistance of the stack terminal short circuit. Stack terminal short circuits of suitably high resistance may not be detrimental to the normal operation of the system, and so a maximum acceptable rate of discharge may be predetermined, corresponding to a minimum acceptable resistance of a stack terminal short circuit.
To detect a stack terminal short circuit, a so-called ‘selected voltage reading’ technique can be employed. The selected voltage reading technique involves determining the voltage VB at the bias point PB with an injector 12a or 12b selected, i.e. with an injector select switch SQ1 or SQ2 closed. When an injector select switch SQ1 or SQ2 is closed, the voltage VB measured at the bias point PB is related to the voltage on the selected injector 12a or 12b. The voltage on the selected injector 12a or 12b can be obtained by subtracting the voltage on the mid voltage rail VM (55 V in this example) from the voltage VB at the bias point PB.
In the selected voltage reading technique, the voltage measurement is performed after a predetermined period following a charge event on the bank 33. The voltage on an injector 12a, 12b at the end of a charge event is known. If the voltage VB at the bias point PB is less than a predetermined voltage level, then this is indicative of a stack terminal short circuit, having a resistance below a predetermined minimum acceptable value, on one or both of the injectors 12a, 12b. It should be appreciated that the expression ‘voltage on an injector’ is used for convenience and refers to the voltage on the piezoelectric stack of the injector actuator 16a, 16b. 
A disadvantage of using the selected voltage reading as described above to determine stack terminal short circuits on the injectors 12a, 12b, is that this technique can entail a charge share between the injectors 12a and 12b in the event of a stack terminal fault. Charge sharing occurs when a non-faulty injector 12a, 12b is selected causing it to discharge into a faulty injector 12a, 12b. 
For example, referring to FIG. 2, if the second injector 12b has a stack terminal short circuit, then selecting the first injector 12a by closing the first injector select switch SQ1 will result in a closed loop in the injector bank circuit 33. The closed loop includes the diode D2 connected across the second injector select switch SQ2, and the closed first injector select switch SQ1. An uncontrolled current will flow from the non-faulty first injector 12a, around the closed loop to charge the discharged faulty second injector 12b, in turn resulting in the non-faulty first injector 12a discharging. Charge sharing can also occur if one of the injectors 12a, 12b has a stack terminal short circuit, when an injector 12a or 12b is selected for discharge by closing the associated injector select switch SQ1 or SQ2. Whilst the selected voltage reading technique is able to determine stack terminal short circuit faults on the injector bank 33, charging sharing prevents this technique from being able to determine which of the individual injectors 12a, 12b is at fault.
An alternative diagnostic technique for detecting stack terminal faults is a so-called ‘charge pulse’ technique, as described in EP 06256140.2 and EP 07252534.8. The charge pulse technique comprises performing a first ‘charge pulse’ on the injectors 12a and 12b by closing the charge switch Q1 for a short period of time; opening the charge switch Q1 and allowing a predetermined period of time to elapse before closing the charge switch Q1 again for another short period of time to perform a second charge pulse on the injectors 12a, 12b. If either of the injectors 12a, 12b has a stack terminal short circuit, then it will discharge to an extent during the predetermined period prior to the second charge pulse being performed. Hence, when the second charge pulse is performed, a current will flow in the charge circuit to recharge the discharged faulty injector 12a or 12b. 
If neither of the injectors 12a, 12b has a stack terminal short circuit, then both injectors 12a, 12b should substantially hold their charge during the predetermined period prior to the second charge pulse being performed, in which case substantially no current will flow in the charge circuit when the second charge pulse is performed. The current sensing and control means 35 is arranged to monitor current flow during the second charge pulse. The presence of a current during the second charge pulse above a predetermined threshold current level is indicative of a stack terminal short circuit on one or both of the injectors 12a, 12b on the bank 33. The predetermined threshold current level is based on a minimum acceptable resistance of stack terminal short circuit and the duration of the predetermined period prior to the second charge pulse being performed.
Whilst the charge pulse technique described above does not suffer from the charge share problems of the selected voltage reading technique (because both injector select switches SQ1, SQ2 remain open), in common with the other diagnostic techniques described above, the charge pulse technique is not able to determine which of the individual injectors 12a, 12b is at fault.
As mentioned above, in each of the diagnostic techniques described above, faults can be traced as far as the injector bank 33, but faulty injectors 12a, 12b cannot be identified. In such circumstances, the recovery action on detection of a fault is to shut down the entire injector bank 33. In the case of a four-cylinder engine, this would result in the engine running on only two cylinders, when the fault may only be associated with one of the injectors 12a, 12b on the bank 33. This can cause associated problems at engine service, because further tests must be performed to identify the injector 12a, 12b at fault.